Infrared (IR) radiation absorbing compounds are beneficial for many applications. They are useful in optical recording media, thermal writing display, laser printing, laser filters, IR photography, medical applications, plastic reheat, laser welding, laser marking and for protective goggles for welding and for protection from lasers in military applications. Several classes of IR absorbing compounds are known including carbon black, inorganic pigments, metals, organic dyes, and organic pigments.
Many classes of organic IR dyes are known including squaric acid dyes, croconic acid dyes, imminium dyes, cyanines, anthraquinones, quaterylenes, perylenes, porphryins, triphenylmethanes, polymethines, quinine dyes, azos, and others. Organometallic IR absorbers are also known including metal phthalocyanine, metal naphthocyanine, and metal dithiolenes. A thorough review of IR absorbing compounds has been published (Chem. Rev. 1992, 92, 1197-1226).
Infrared radiation is commonly defined as “light” with wavelength greater than 700 nm and less than 1 mm. Near IR (NIR) light is commonly defined as radiation with a wavelength between 700 nm and 14,000 nm. The majority of common NIR absorbing dyes and pigments have maximum IR absorbance between 700 nm and 3000 nm. Most common NIR dyes absorb between about 700 nm and 1500 nm. Compounds such as carbon black, which are black-body absorbers, absorb over a very broad wavelength range.
It is highly desirable for NIR absorbing compounds to have strong NIR absorbance yet have minimal visible light absorbance (i.e., absorbance between 400 nm and 700 nm). These compounds are advantageous for use in non-colored and lightly colored transparent applications where strong NIR absorption without strong color is desired. Some advantageous NIR compounds are mostly transparent to visible light in liquid or plastic compositions.
To be highly useful in many applications, a NIR absorber must have several properties in addition to absorption at the desired wavelength. The absorption efficiency must be sufficiently strong (as characterized by the molar absorptivity constant). The compound(s) should be sufficiently soluble or compatible in the medium in which it is used. Further, the compound should be sufficiently thermally and oxidatively stable to facilitate incorporation and use in the desired application without excessive degradation during the desired lifetime of the absorber.
Transmission laser welding is a well known method for joining thermoplastic articles. In one embodiment of this technique an IR laser beam is passed through an IR transparent part and impinges on a second part containing IR absorbing species. These IR absorbing species can be contained in the bulk of the polymer through melt mixing or applied to the interface between the two parts as a coating. The IR absorbing species absorbs sufficient energy from the laser to melt the surrounding plastic. The molten plastic from the two parts flows together and cools to form a strong joint or weld. It is necessary for the IR absorbing species to be thermally stable enough to survive for a brief period of time in the molten polymer. A higher level of thermal stability is needed for incorporation of the IR absorbing compound into the molten polymer during the fabrication of the article. It is particularly advantageous for the IR absorbing compound to impart minimal color. in or on the resulting part, although this is not always necessary. Thus, there is a need in this industry for compounds that will afford the correct absorption characteristics, and also exhibit a high level of thermal stability, while imparting minimal color upon welded parts or seams. Few NIR dyes have the correct combination of properties for incorporation into thermoplastic media or to be used for laser welding via a coating. Of the few that are known, cyanine, imminium/diimonium, quaterylenes, metal dithiolenes, and squaryliums are most common. Many of these compounds are difficult and expensive to make.
There exists therefore a strong need for new and improved IR absorbing compounds. There is a need in the industry for NIR absorbing compounds that are easy to manufacture, with sufficient thermal stability for incorporation into plastic. Such compounds may be used for many different applications, including for example for laser welding through a coating. It would be desirable to incorporate compounds that provide a relatively low visible absorption and impart minimal color to articles in which they are incorporated.